Navien NCB-240E: does the 3-way valve take the place of a check valve?

[Tripp Knightly]

I have an NCB-240E combi whose domestic hot is flaky (at best now, cold showers abound). Heating seems ok. I’ve seen on other Navien units that intermittent domestic hot could be fixed easily / cheaply with a replacement check valve. Am I right that on this unit it’s instead of the check valve the servo-actuated 3-way valve? Price isn’t terrible but sheesh looks like a pain to swap.

I also understand it could instead just be the flow sensor. I’m not sure if there’s a tell-tale way to know which?

Finally I understand that, regardless of which it is presently, that 3-way fails too often so it’s in my future.

EDIT/UPDATE: I replaced the flow sensor and the problem seems fixed for now. Still want to know how you know which part is the problem.

 

[Fitter30]

How long has it been since the last cleaning with vinegar? Yearly maintenance?

 

[Sponsor]

 

[Tripp Knightly]

Not mine. No yearly maintenance. Maybe 7 years old. When I did the work yesterday I cleaned the debris trap (some silt!), air intake (nearly fine), water intake screen (nearly fine).

I’m going to de-scale once I get some confirmation on the max HP transfer / utility pump I can use (have a 1/3hp, that might be too much) as well as whether regular 5% household vinegar needs dilution.

 

[Tripp Knightly]

Navien told me: 5% household vinegar ok, pump should be very low power, can be 1/4 to 1/8hp. I had heard / read 1/6. Mine is 1/3 but I think I can make it weaker with a rheostat / speed control you’d hook up to a power tool.